Sensory processes begin with stimuli stimuli are forms of energy internal or external Receptors alert nervous systems (most animals) use action potentials (membrane voltage) results in a ‘sensation’ After a stimulus is processed, a response may be generated
Neurons = sensory, interneurons, and motor neurons Fundamental unit of the brain and nervous system** Neuron structure is all about form and function Most organelles located in the cell body Highly branched extensions, called dendrites, receive signals Neurons have a single axon to transmit signals transmit pulses of electrical current terminals connect nerve cells
Sensory pathways have four basic functions: reception, transduction, transmission, perception Sensory pathway starts with stimuli (1) Sensory reception: sensory cells are specialized neurons located inside or on outside of body detect specific types of stimuli
Transduction: Stimulus receptors change flow of ions across nerve cell membranes opens/closes ion membrane channels along axon Cl-, Na+, K+ Membrane potential: gradient that forces ions to passively move in one direction can be processed by central nervous system at rest, inside of cell is more negative than outside magnitude of change in ion flow = strength of stimulus
Transmission: Action potential: caused by change in membrane potential travels along axon and flows to other cells Receptor generates action potentials that travel to central nervous system large stimulus = more action potentials Speed of action potential depends on species length of axons varies vertebrate axons are insulated
Perception: How does the brain know the difference in stimuli? action potentials are not different from each other Action potentials reach brain/spinal cord via dedicated neural pathways brain distinguishes stimuli on the path they arrive 100 million neurons with 100 trillion connections = human brain actions potentials sent back to target receptors via motor neurons
Response to touch, stretch, motion and sound Receptors sense physical deformation Ion channels linked to structures that end outside cell hair or cilia bending/stretching structure generates response Touch receptors often embedded in tissue
(1) Transmit information about solute concentrations detect change in blood solutes → stimulate thirst (2) Respond to individual kinds of molecules glucose, O2, CO2, amino acids, pheromones Stimulus molecule binds to a chemoreceptor silk worms detect pheromones from females several kilometers away Chemoreceptor becomes more or less permeable to ions
Receptors that detect light, electricity and magnetism Animals create & detect electromagnetic energy to hunt Used by animal kingdom to migrate proteins can sense Earth’s magnetic fields insects, birds & mammals
Receptors that detect light, energy and magnetism may also create & detect electromagnetic energy Used widely by animals to sense prey 16% of fish species use electroreceptors to detect bioelectric fields from prey
Receptors detect heat and cold Hot pepper science has really helped!!! Receptors that respond to capsaicin also respond to high temperatures taste ’hot”: activate same receptors as hot soup opposite for ‘cold’ flavor Mammals: thermoreceptors for specific temperatures Venomous snakes: infrared receptors to detect ‘warm’ prey
Pain receptors (nocioreceptors) detect stimuli that reflect harmful conditions damaged or inflamed tissues gene mutations can cause chronic insensitivity to pain (C.I.P.) by making nocioreceptos non-functional Respond to damaged/inflamed tissues perceived as pain trigger defenses (withdraw)